Wi-Fi, also known as Wireless Local Area Network (WLAN), is a technology that currently mainly operates on the 2.4 GHz or the 5 GHz band. There are specifications regulating an access points' or wireless terminals' physical (PHY) layer, medium access layer (MAC) layer and other aspects in order to secure compatibility and inter-operability between different WLAN entities, e.g. between an access point and mobile terminals, both of which may be referred to as stations (STAs) in the following. Wi-Fi is generally operated in unlicensed bands, and as such, communication over Wi-Fi may be subject to interference sources from any number of known and unknown devices. Wi-Fi is commonly used as wireless extensions to fixed broadband access, e.g., in domestic environments and hotspots, like airports, train stations and restaurants.
The WLAN technology relies on a procedure known as Carrier Sensing Multiple Access with Collision Avoidance (CSMA/CA) in order to efficiently and in a fair way share the wireless medium among the different WLAN entities (STAs) and among different Radio Access Technologies (RATs). The CSMA/CA procedure applied in the WLAN system demands that each entity that wishes to send data senses the common communication channel before performing a transmission. Such sensing, or scanning, is done in order to avoid that simultaneous transmissions are made, which usually results in loss of data and need for retransmissions.
FIG. 1 illustrates, in a simplified manner, a clear channel assessment (CCA), applied by a STA that has a frame to transmit (box 1). In order to assess if a channel is busy, the STA starts scanning (box 2) the channel. If the STA detects a transmission, it establishes the received signal strength thereof. If the received signal strength is higher than a pre-determined threshold, referred to as Clear Channel Assessment threshold (CCAT), then the STA deems the channel as busy and defers its own transmission (box 3). If the STA does not detect any such received signal strength, it assumes the channel to be idle and transmits the frame (box 4).
FIG. 2 illustrates an example of a Basic Serving Set (BSS) comprising an access point AP and a number of user entities (indicated STA A, STAB in the figure) located within a coverage area C1 (also denoted cell) being served by the AP. When, for instance, the access point (AP) has something to transmit, it performs the CCA procedure. All other transmitting entities STA A, STAB within its service area C1 will then, at least in a simplified propagation environment, cause the AP to defer its transmissions. It awaits a random back-off time period and then performs the CCA again.
In current systems a common CCAT is defined for each AP to be used when performing channel sensing for transmission to any of the entities STA A, STAB, associated to the AP. The entities STA A, STAB also use this common CCAT as criterion for deciding whether or not to transmit a frame.
With such static CCAT, a STA (APs and other entities) may refrain from accessing the wireless medium since it is also exposed to transmissions in neighboring AP areas, although simultaneous transmissions would be possible without increased risk of lost packets and subsequent retransmissions. This limits the performance of the current systems, especially as the CCAT used today is set rather conservatively to −82 dBm.
The use of dynamic thresholds has therefore been discussed. If the STAs could adapt their carrier sensing threshold dynamically then the amount of simultaneous transmissions in the system might be increased without significantly increasing the probability of collisions within and between the different areas.
Referring again to FIG. 1, when an entity has a frame to transmit (box 1), it again assesses if a channel is busy by scanning (box 2) the channel. However, in the case of a dynamic CCAT, instead of the fixed CCAT (exemplified by −82 dBm in the figure) the entity uses the dynamically set CCAT for establishing whether the received signal strength level is such that the entity should defer its transmission or perform the transmission. Various suggestions have been presented on how to set the dynamic thresholds. Some studies have shown that adjusting the CCAT to a more aggressive value provides throughput increases for both the mean and 5th percentile user throughputs.
Even though the dynamic adjustment of the CCAT would, in many scenarios, lead to improved system performance in terms of spectrum usage and system throughput, there is a tendency of also increasing the number of failed transmissions. The reason for this is that when a certain STA (either AP or non-AP STA) attempts to use the channel by applying a more aggressive channel access mechanism and then transmit simultaneously with an already on-going transmission, it creates additional interference which might lead to losses for the on-going transmission and resulting in re-transmissions.
There is a need for improving system throughput and spectrum usage by enabling simultaneous transmissions while still keeping the number of re-transmissions to a minimum.